1. Field of the Invention
The invention relates to new polymeric materials that comprise pyridine and/or tetramethyl biphenyl moieties. Preferred polymeric materials of the invention can exhibit high glass transition temperature (e.g. >200° C. such as up to 280° C.), high thermal and oxidative stability (e.g. >300° C. or 400° C. such as up to 450° C.) doping such as with phosphoric acid can result in high acid uptakes in preferred systems.
Following the materials characterization with conventional techniques, membrane electrode assemblies were constructed in order to study their fuel cell performance. The prepared MEAs were tested in a single cell at temperatures up to 170° C. The long term stability of the system was studied by measuring the current output at a constant voltage of −500 mV for 1000 h.
2. Background
Polymer electrolyte membrane fuel cells (PEMFCs) operating at 90° C. are currently the best candidates for use in stationary and automobile applications. Up to now Nafion, has been applied almost exclusively as polymer electrolyte. However, its conductivity is dependent on the presence of water demanding thus humidification of the feed gases while limiting the cell operation temperature to be below 100° C. At that temperature range the presence of impurities such as carbon monoxide in the hydrogen will have poisonous effect on the electrocatalyst. Even though new electrocatalysts have been developed for a typical operational temperature of 80° C., 50-100 ppm of carbon monoxide can deactivate the catalyst. The need for humidified gases as well as the demand high purity hydrogen increase the operation cost sufficiently.
Operation of the fuel cell at temperatures above 150° C. offers certain advantages such as increased catalyst activity, decreased susceptibility of the anodes catalyst to poisoning due to impurities in the fuel cell stream, easier thermal management than conventional PEM fuel cells. The basic prerequisites for a polymer to be used as high temperature electrolyte is thermal and oxidative stability, excellent mechanical properties combined with high proton conductivity after doping with a strong acid. Besides polybenzimidazole which is a well established high temperature polymer electrolyte, there is a significant effort towards the development of some novel polymeric materials which fulfill the above requirements.
Various attempts have been made to improve the mechanical properties of PBI by using polymer blends composed of PBI and a thermoplastic elastomer (Macromolecules 2000, 33, 7609, WO Patent 01/18894 A2) in order to combine the acid doping ability of the PBI with the exceptional mechanical properties of the thermoplastic elastomer. Additionally, blends of PBI with aromatic polyether copolymer containing pyridine units in the main chain have also been prepared, resulting in easily doped membranes with excellent mechanical properties and superior oxidative stability (Journal of the Membrane Science 2003, 252, 115). Certain efforts also have been made to develop low cost polymeric systems that will combine all the desired properties for application in fuel cells operating at temperatures above 150° C.